1. Field of the Invention
The invention relates generally to the field of subsurface formation evaluation using well logging measurements. More specifically, the invention relates to methods for determining particle size distribution of rock formations using nuclear magnetic resonance (“NMR”) well logging measurements.
2. Background Art
Wellbores are drilled through subsurface rock formations for, among other purposes, extraction of useful fluids such as oil and gas from porous, permeable rock formations penetrated by such wellbores. The porous formations include rock mineral grains of various shapes and sizes, wherein the grains are bound to each other (cemented) in varying degrees depending on the post depositional history of the particular rock formation. The fluids are contained in the pore spaces. A wellbore is said to be “completed” when hydraulic connection is made between a formation that is intended to produce fluid and the Earth's surface using various conduits and flow control devices.
Particle size distribution (PSD) of rock formations, that is the distribution of sizes of the rock grains, is a physical parameter that is important for the selection of the completion type, sand screen type and opening size, and gravel size (if needed) of a “gravel pack” for control of formation solids when fluids are produced. PSD can be obtained from sieve analysis or laser particle size analysis of the rock formation from produced samples, bailed sample, side wall core or conventional core.
A sample of the rock formation can be easily obtained from a producing well in which a substantial volume of sand moves into the wellbore. However, the produced sand sample will generally have a higher percentage of fine-grained sand than what is originally present in the rock formation. This is because coarse sand particles tend to fall, rather than move upward to the surface, and settle at the bottom of the well when the sand moves into the wellbore. For the same reason, a bailed sample will generally have a higher fraction of coarse sand than what is present in the reservoir rock. Sand samples obtained from sidewall (percussion or drilled) cores can also give misleading results, particularly in the case of percussion sidewall cores. When the sample taking projectiles strike the face of the formation, they can crush the rock grains, generating more fine particles than may be present in the undisturbed rock formation. The sidewall core sample could also contain drilling fluid (“mud”) solids that can be misidentified as formation material. The most representative formation sample is obtained from conventional (drilled) cores. However, such samples are not readily available in most cases due to cost of coring operations. If drilled core samples are available, small plugs can be taken out of the core at various longitudinal positions along such sample for a complete and accurate grain size distribution.
There is a need for other ways of obtaining particle size distribution of subsurface rock formations without the need to retrieve actual formation samples.